Polymer polyols in the context of this invention are understood to be products which are obtained by the polymerization of ethylenically unsaturated compounds in high molecular weight polyols. These may be used, for instance, to produce polyurethane flexible foams. The ethylenically unsaturated compounds mainly used are the monomers styrene and acrylonitrile which are radically polymerized in polyether-polyols.
The preparation of this type of polymer polyol is described in, for example, U.S. Pat. Nos. 3,383,351, and 3,523,093, or German Auslegeschrift 2,837,026.
The most important objectives when preparing polymer polyols are to achieve high solids contents (at least 40%) for high proportions of styrene (at least 60 parts by weight of monomers), the lowest possible viscosity and excellent product stability at the same time.
In order to achieve this product stability, i.e. to prevent the build-up of undesired agglomerated polymer particles being deposited out of the continuous phase, the polyether polyol, the polymer particles must be stabilized during preparation of the polymer polyol. This stabilization can be achieved by also polymerizing a compound which contains ethylenically unsaturated, polymerizable groups, and which is compatible with the polyether polyol phase, as a stabilizer, to form so-called macromonomers or macromers. These macromers copolymerize with the vinyl monomers such that the polymer particles produced are sterically stabilized by means of side chains and are thus protected against agglomeration and deposition.
The production of polymer polyols by incorporating macro-monomers is described in, for example, U.S. Pat. Nos. 3,850,861, 3,823,201, 4,689,354, 4,342,840, and Reissue 33,291. The ethylenically unsaturated double bonds are introduced into polyether polyols, for example, by reaction with cyclic, unsaturated acid anhydrides such as maleic anhydride, followed by reaction with ethylene oxide; by reaction with an unsaturated epoxide such as allylglycidyl ether. Japanese Patent 61,115,919 describes polyols which are obtained by incorporating macromonomers with a molar mass of 700 to 3500 g/mol, which are obtained, for example, by reacting TDI, hydroxyethyl methacrylate and a polyether polyol in equimolar proportions.
U.S. Pat. No. 4,390,645 describes the preparation of macro-monomers by reacting polyalkylene oxides with unsaturated, radically polymerizable isocyanates such as 2-isocyanatoethyl methacrylate.
Even though all the methods mentioned lead to polymer polyols which basically can be used as polyol components in polyurethane flexible foams, further improvements are desirable. Some methods for preparing the macromonomers are complicated, the viscosities mentioned in the literature are often relatively high and small agglomerated polymer particles are frequently observed when discharging the products, i.e. the stability of the products, as can be seen from the comparison examples, is not adequate.
The basic object of the invention is, therefore, to provide a new process for preparing low-viscosity, stable, agglomerate-free polymer polyols. This object can be achieved using the invention described in more detail in the following.